The image quality of a cathode ray tube greatly depends on the performance of the electron gun, and especially, the focus characteristic of electron beams made by means of an electron gun is an important factor which directly affects the image quality. That is, a good electron gun will form the smallest beam spots as far as possible with low spherical aberration, and such an electron gun having the above mentioned purposes has been developed in various types. Among them, there is one in which, in order to reduce the spherical aberration, the beam passing holes of the main lens are overlapped in such a manner that three beam passing holes should be continuously formed. But in this type, if the beam current is increased, then the spherical aberration may be increased. In order to settle said apprehension, the length of the electrode G3 has to be increased enough for the magnification of the main lens to be reduced. However, such case, there is a problem of practicability.
In consideration of these circumstances, Korean Patent publication No. 87-281 discloses an improved electron gun based on a new conception, which is characterized in that the spherical aberration can be reduced to a great degree by providing two unipotential type auxiliary lenses at a position upstream of the bipotential type major lens. That is, the electron beam which had been focused by means of a single main lens in the preceeding technology has come to be focused through two auxiliary lenses and a major lens. Such a dispersion of the focusing steps over three lenses is for reducing the spherical aberration by means of low magnification lenses. That is, while the beam is focused by a multistep mechanism, the distance of the virtual object point (the starting point of the beam) is separated farther from the screen step by step, so that the incident angles of the electron beam relative to the respective lenses should be reduced step by step.
The electron gun based on the above described principle provided a satisfactory performance to a certain extent, but in order to obtain the maximum performance, the dimensions of the components and voltages applied to them should be more thoroughly deliberated.